Bone Tissue & Skeletal System: Structure, Function, and Clinical Relevance

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Bone Tissue & Skeletal System

Overview

The skeletal system provides the structural framework for the human body, supports movement, protects internal organs, and serves as a reservoir for minerals. Bone tissue is dynamic, constantly undergoing growth and remodeling in response to various factors.

Bone Growth and Remodeling

Influencing Factors

Bone growth and remodeling are influenced by three primary factors: exercise, nutrition, and hormones. These factors interact to maintain bone strength, density, and overall health throughout life.

Effect of Exercise on Bone

Role of Physical Activity

Regular physical activity is essential for bone health. Mechanical stresses applied to bones during exercise stimulate bone-forming cells and promote bone deposition.

Bones are remodeled throughout life: Bone tissue is continuously broken down and rebuilt, adapting to the mechanical demands placed upon it.
Exercise stimulates osteoblasts: Osteoblasts are cells responsible for bone formation. Increased activity leads to greater bone density and mass, especially in athletes and manual workers.
Bone loss due to inactivity: Lack of physical activity can result in rapid bone degeneration. Up to one-third of bone mass may be lost within a few weeks of inactivity.

Key Concept: "What you don’t use, you lose." Stresses applied to bones during physical activity are essential to maintain bone strength and mass by stimulating osteoblasts and bone deposition.

Effects of Nutrition on Bone

Essential Nutritional Factors

Normal bone growth and maintenance require adequate nutrition, including minerals and vitamins.

Minerals: Calcium (Ca2+) and phosphorus (P) are critical for bone structure. Small amounts of magnesium, fluoride, iron, and manganese are also necessary.
Vitamins: Vitamins A, C, and D (calcitriol) play important roles in bone health. Vitamin D is especially crucial for calcium absorption.

Effects of Hormones on Bone

Hormonal Regulation

Hormones regulate bone growth, remodeling, and mineral homeostasis.

Calcitonin: Produced by the thyroid gland; lowers blood calcium levels by inhibiting osteoclast activity.
Parathyroid Hormone (PTH): Produced by the parathyroid glands; increases blood calcium levels by stimulating osteoclasts and increasing calcium reabsorption in the kidneys.
Growth Hormone (GH): Produced by the pituitary gland; stimulates bone growth.
Thyroid Hormones (T3 & T4): Produced by the thyroid gland; regulate metabolism and bone growth.
Insulin: Produced by the pancreas; promotes bone formation.
Sex Hormones: Estrogen (ovaries) and testosterone (testes) stimulate bone growth and maintenance.

Calcium Homeostasis

Importance of Calcium Regulation

Calcium ions are vital for neural communication, muscle contraction, and many other physiological processes. The body tightly regulates blood calcium levels through hormonal control.

Hypocalcemia: Deficiency of blood calcium; leads to nervous system excitability, muscle spasms, tremors, tetany, and potentially laryngospasm and suffocation.
Hypercalcemia: Excess blood calcium; causes muscle weakness, sluggish reflexes, and risk of cardiac arrest.

Hormonal Control of Calcium

Calcitriol (Vitamin D): Synthesized from vitamin D precursors in the skin and liver, then activated in the kidneys. Raises blood calcium by increasing absorption from the digestive tract.
Calcitonin: Lowers blood calcium by inhibiting osteoclasts and increasing calcium excretion by the kidneys.
Parathyroid Hormone (PTH): Raises blood calcium by stimulating osteoclasts, increasing kidney reabsorption, and enhancing intestinal absorption.

Bone Fracture and Repair

Types of Fractures

Closed (simple) fracture: Bone breaks but does not penetrate the skin.
Open (compound) fracture: Bone breaks and penetrates the skin.
Transverse fracture: Bone breaks at a right angle to its axis.

Stages of Healing

Fracture Hematoma: Blood vessels rupture, forming a clot.
Soft Callus: Fibroblasts produce collagen fibers and chondroblasts produce hyaline cartilage.
Hard Callus: Osteoblasts produce new bone, ossification occurs.
Remodeling: Spongy bone is replaced by compact bone, restoring shape and strength.

Healing Time: Typically 8-12 weeks, longer in elderly individuals.

Articulations & Joints

Definition and Function

An articulation (joint) is a structural arrangement connecting two or more bones at their sites of contact. Joints enable body movement and provide stability.

Classification of Joints

Functional Classification:
- Synarthrosis: No movement (e.g., sutures in the skull).
- Amphiarthrosis: Slight movement (e.g., joints between pubic bones).
- Diarthrosis: Free movement (synovial joints).
Structural Classification:
- Fibrous Joints: Bones joined by fibrous tissue (e.g., syndesmosis, sutures).
- Cartilaginous Joints: Bones joined by cartilage (e.g., synchondrosis).
- Synovial Joints: Bones separated by a synovial cavity, allowing free movement.

Synovial Joints

Basic Structure

Synovial joints are the most moveable type of joint, found at the ends of long bones. They are characterized by the presence of a synovial cavity filled with synovial fluid.

Articular Cartilage: Hyaline cartilage covers bone surfaces, providing a smooth, low-friction surface and shock absorption.
Ligaments: Dense regular connective tissue that stabilizes joints and prevents excessive movement.
Joint Capsule: Composed of an outer fibrous layer and an inner synovial membrane, which produces synovial fluid.
Synovial Fluid: Yellowish, viscous fluid that lubricates, nourishes cartilage, and absorbs shock.

Accessory Structures

Menisci: C-shaped cartilages in the knee that absorb shock, guide movement, and distribute forces.
Accessory Ligaments: Support and strengthen joints; torn ligaments can lead to instability.
Bursae: Saclike extensions of the joint capsule filled with synovial fluid, reducing friction between structures.
Pads: Superficial to the joint capsule, protect articular cartilages.
Tendons: Attach muscles to bones, supporting joint stability.

Range of Motion

Determined by the structure of articular surfaces, strength of ligaments/tendons/capsule, and muscle action.
Stretching ligaments increases range of motion; "double-jointed" individuals have longer or more flexible ligaments.
Nervous system monitors joint position and muscle tone.

Clinical Applications

Joint Injuries

Dislocation (luxation): Articulating surfaces are forced out of position, damaging cartilage, ligaments, and joint capsule.
Subluxation: Partial dislocation.

Aging and Joints

Decreased synovial fluid production.
Thinner articular cartilage.
Shorter and less flexible ligaments.
Genetic factors and "wear & tear" contribute to joint degeneration.

Common Clinical Problems

Rheumatism: Inflammatory conditions affecting muscles, joints, or fibrous tissue, causing pain and stiffness.
Osteoarthritis: Degeneration of articular cartilage due to wear and tear or genetic factors; accompanied by pain and crepitus (crackling sounds).
Rheumatoid Arthritis: Autoimmune attack on synovial membrane, leading to cartilage and bone degradation, joint deformity, and inflammation.
Gouty Arthritis: Crystals (uric acid or calcium salts) form within synovial fluid due to metabolic disorders.
Joint Immobilization: Reduces synovial fluid flow, can cause arthritis symptoms; treated by physiotherapy and continuous passive motion.

Summary Table: Hormonal Regulation of Calcium

Hormone	Source	Main Effect on Calcium	Action on Bone	Action on Kidneys	Action on Digestive Tract
Calcitriol (Vitamin D)	Kidneys (activated from skin/liver precursors)	Raises blood Ca2+	Promotes bone resorption	Increases reabsorption	Increases absorption
Calcitonin	Thyroid gland	Lowers blood Ca2+	Inhibits osteoclasts	Increases excretion	No direct effect
Parathyroid Hormone (PTH)	Parathyroid glands	Raises blood Ca2+	Stimulates osteoclasts	Increases reabsorption	Increases absorption

Key Equations

Calcium Homeostasis:

Additional info: Academic context and terminology have been expanded for clarity and completeness.